The P21 test method has been in existence almost unchanged since 1979, with a supplement published in 1991 to reflect the change from working stress design to limit states design. It is cited in NZS 3604 Timber framed buildings and is still in regular day-to-day use to determine wind and earthquake ratings of bracing elements generally within the scope of NZS 3604.

With the imminent publication of an updated revision to NZS 3604, it was decided to update the P21 method to incorporate current best practice, remove ambiguities, align with the new loadings standard AS/NZS 1170 and prepare for citation in NZS 3604. There is a new section covering the application of brace ratings, including its use in specifically designed buildings.

The resulting document is the culmination of input from the members of the NZS 3604 review bracing subcommittee and researchers at BRANZ.